Introduction

Relays play a significant role in electrical control systems. There are many types of relays like electromagnetic relays, time relays and thermal relays. Solid state relays (SSRs) are one of them. They can supply/cut-off power to electrical appliances using a low voltage control signal. In this article, we’ll be discussing SSRs, how they work and their applications.

Introduction To Relay Types

There are times where we need to control a high voltage/high current electrical equipment using a small signal. For example, imagine a scenario where we need to control a 220V single phase 1HP motor using a small switch/button or using a PLC. In an application like this, the motor cannot be connected directly to the PLC or the switch. This not only exceeds the switch’s rating, but also poses an electrical hazard due to high voltages and current being handled.

Relays are useful in such instances. A relay allows us to control a large appliance which draws higher current (such as a 220V 20A load) using a small voltage signal (24V, 100mA). A relay also provides electrical isolation between the high voltage side and the low voltage side. Relays are on/off type devices that have only those two distinct modes of operation.

Before we move onto solid-state relays, let’s go through some of the most popular types of relays available in the market to understand how a regular relay works.

There are many types of relays available in the market. In general, they are electrically controlled on/off switches that have single/multi-pole and throw setup.

Here are some of the popular relay types and their functionality in brief:

• Electromagnetic/Electromechanical relays
  • These are the most popular and generic types of relays. They consist of a mechanical arm that makes/breaks contact with the contact conductive terminals of the relay, and it is actuated by applying a voltage to the built-in coil. Electromagnetic relays can control both AC and DC appliances. Electromagnetic relays are available in various coil voltages and contact ratings.
• Small signal relays
  • Small signal relays are mostly found in automotive and industrial automation applications. These are miniature versions of electromechanical relays that switch low voltage, low current signals such as PLC digital output signals.
• Time-delay relays
  • Time delay relays consist of a built-in timer and an electromechanical relay to delay turning on after applying a turn-on signal. These are found in motor control circuits to start high power motors.
• Polarized relays
  • Polarized relays are a special type of relay that are sensitive to the direction of current applied. When a DC current is applied to the coil in a certain polarity, the relay switches to a certain position, activating a certain set of contacts. When the polarity is swapped, it activates another set of contacts. When power is removed, some polarized relays return to a ‘neutral position’ to break all contacts.

The other most popular relay type is the Solid State Relay. With the understanding about the relays we’ve got so far, let’s discuss solid state relays.

What Is A Solid State Relay?

A Solid state Relay (also known as SSR) is another type of relay that operates from a small AC/DC input signal. SSRs work very similarly to EMRs (Electro-Meahcnical Relays). However, they do not have the moving components. SSRs instead use electrical and optical components (which gives the name solid-state) to perform the switching task and keep the input signal isolated from the switching side.

Similar to an electromechanical relay, solid-state relays also provide near-infinite contact resistance/impedance when open and near-zero resistance/impedance during operation. Depending on the internal construction of the control circuitry, SSRs can control either AC, DC or both types. This is possible due to the variety of semiconductor choices available as the power electronics. Solid-state relays can be designed using SCRs, TRIACs or even transistors/MOSFETs.

One of the key things that differentiates a SSR with its electromechanical counterpart is the operational life. Electromechanical relays have a very limited contact lifecycle because they physically engage/disengage the contacts. This causes electrical arcs to generate between the opening contacts which degrades the contact surface. While the heavy duty relays are designed to counter this, they are not permanently immune to the wear and tear.

SSRs, on the other hand, are fully solid state and have zero moving parts. This allows them to last thousands of switching cycles under rated load without having to worry about the operational stability. This also improves the switching speed of the SSR.

Solid State Relay Circuit

Solid state relays are simple devices from a usability point of view. They have control signal input, and a switched output that controls high power electrical loads. Their internal construction is far more complicated that what meets the eye. Let’s discuss the SSR circuit and how it works.

As mentioned before, solid state relays offer electrical isolation between the control signal side and the load side. Similar to electromechanical relays where the isolation happens through physically separated contacts, SSRs achieve this by optically isolating the input signal.

This is done using a special semiconductor device called ‘optocoupler’ (also known as ‘optoisolator’.). Optocouplers contain one or more infra-red emitting diodes or LEDs along with a photosensitive device to provide optical signal isolation.

When the control signal is provided (very low DC voltage in the range of 2-3V), it turns on the IR LED built-in to the SSR. The emitted beam is received by the photosensitive device to activate the output. The photosensitive device is placed farther from the emitter and to provide the electrical isolation. With this implementation, an SSR can easily switch a 220V AC load with a control signal as low as 5V DC.

The control signal can originate multiple ways. It can be either,

• Solid state DC signal
  • Solid state DC signals can be originating from simple switches or direct power sources like battery cells.
• Digital output signal
  • Controllers such as microcontrollers or microprocessors, PLCs can also generate signals that can be fed into SSRs for controlling loads.
• Logic gate signals
  • For applications that do not require the processing power of a microcontroller, a combinational logic gate circuit’s output can be connected to an SSR for turning a load on/off according to a set of conditional inputs.

Types Of SSRs

There are many types of solid-state relays. They differ from each other by the functionality. The operating principle is very similar, although they are used in different applications.

Instant Switching SSR

Instant switching solid-state relays switch the output on immediately when a control voltage is applied. These SSRs have atypical response time less than 1 milliseconds, making them an ideal component for applications that require fast response and/or phase angle control. These also find applications in inductive load switching.

Instant switching SSRs are usually made of triacs to allow the control of AC signals regardless of the phase angle at the moment of switching. This works identical to a regular switch where the turn on point is random.

Zero Switching SSR

Zero switching, also known as zero-crossing SSRs, turn on at the first zero crossing point of the line voltage regardless of the time control signal is applied. For a 50Hz sinusoidal line voltage, the response time can be between near-zero to 10ms (less than half period).

These SSRs have a special built-in circuitry called ‘zero crossing detector’. When the control signal is applied, this circuit generates a pulse as soon as the AC sinusoidal waveform reaches 0V point. This turns on the triac that controls the load and the triac stays conductive until the line voltage reaches zero again. The cycle repeats as long as the control voltage is applied.

Zero crossing SSRs find their applications in resistive, capacitive and inductive load control systems. The activation at zero-crossing point ensures a minimum surge current flowing into the load during startup.

Peak Switching SSR

Complementary to the zero-crossing type, peak switching SSRs activate the output at the first peak of the line voltage upon applying the control voltage. After this half-cycle, the SSR continues to function like a zero-crossing SSR.

In peak switching SSRs, a zero-crossing detector is coupled with an initial peak detector stage to generate the first turn-on pulse. The SSR does not turn on until the line voltage reaches its peak voltage. As soon as the peak is detected, the load receives power through the triac. When switched at a peak of the supply voltage, inductive loads draw the least amount of inrush current. The use of peak switching SSRs is beneficial in such applications to ensure the load is protected from inrush currents.

Peak switching SSRs are used with heavily inductive loads such as transformers and high power motors.

Analog Switching SSR

Analog switching SSR are a special type of SSRs. They operate with a 4-20mA DC current signal. The output’s phase is proportionally influenced by the input signal. When the control voltage/current signal is removed, the SSR turns off. Analog solid state relays have built-in circuitry that functions as a closed-loop feedback system to control the output voltage as a function of the input voltage.

DC Switching SSR

For resistive and inductive loads, DC switching SSRs are widely used. DC SSRs control the load using MOSFETs of BJTs therefore they are best used with DC loads such as DC heating elements, solenoid valves and DC brushed motors. Since these do not have built-in inductive kickback protection, an external freewheeling diode is required to be connected to the output terminals in reverse bias configuration.

Control Methods

Different types of SSRs have different driving methods. As mentioned above, SSRs require only a small control signal to switch a higher voltage, higher current load. Here are some of the methods used to drive a SSR’s input.

Direct DC Switching

The simplest method of driving an SSR is by applying the control voltage directly to the SSR. For example, if an SSR’s control voltage is 12V DC, directly supplying the voltage signal to the control inputs turns on the SSR. This type of simple implementation can be found in direct-on-line motor control circuits.

Transistor Control

In some cases, the control signal voltage may not be high enough to directly drive the SSR’s inputs. For instance, a microcontroller running at 5V or 3.3V may not be able to provide enough voltage and current to drive the SSR’s internal circuitry. In such cases, the logic voltages need to be translated to a control signal to the SSR’s input. By implementing a circuit similar to the above image, a small input signal can easily control the SSR. The NPN transistor circuit shown above can turn on the SSR when a positive voltage is applied to the base terminal.

Combinational Logic Control

In applications where conditional logic is needed, yet the system is too simple to be controlled by a microcontroller based control system such as a PLC, logic gates can be used. With a circuit similar to the one shown below, the inverted output of a positive combinational logic circuit can directly drive a SSR to control an electrical load.

AC Control Signal

Some systems use only AC power in both control and power electronics. Incorporating an SSR into such a system can be challenging because SSRs are mostly driven using DC signals. However, with the principle of full-bridge rectification, an AC signal at a compatible voltage level can be converted into a rectified DC voltage signal to drive the SSR’s inputs. The figure below shows such an implementation.

However, most of the SSR manufacturers offer AC input solid state relays in their SSR series to overcome this added overhead.

Solid State Relay Advantages

SSRs have many advantages including,

• Long life and high reliability
• Fast response times
• Low EMI
• No contact arcing due to lack of mechanical components
• High resistance to vibration, shock and dust
• Silent operation
• Logic compatibility

However, SSRs have a few disadvantages too:

• Contact voltage drop
  • Since SSRs are made using semiconductor devices, they pose an inherent series resistance even when fully turned on. For example, thyristors can have a voltage drop of 1-1.6V across the terminals. This generates heat, which requires passive or active cooling.
• Transient voltage problems and dV/dt limitations
  • If not implemented properly, SSRs pose the risk of random turn-on caused by regenerative action due to the inherent capacitance present in the semiconductor stages.

How To Choose Right Solid State Relay

When choosing a SSR for a particular application, consider the following key points:

General information

Select a SSR that can handle the rated load current, voltage and the operating temperature. The SSR should generally have a higher rating than the intended application.

Protection features

The SSR should have adequate protection from thermal overload, over-current and transient voltage protection. In many cases, such circuitry should be connected externally.

Also, ensure that the SSR complies with the insulation standards for the application. For example, higher end SSRs have input-to-output insulation resistance of >=4000Vrms AC and output to case insulation resistance of >= 2500Vrms AC

Compliance to industry standards

Selecting an SSR that’s been manufactured to conform with IEC, UL and similar industry standards will ensure the system integrity and reliability.

Conclusion

Solid state relays are excellent switching devices that can replace conventional electromechanical relays in many cases. The initial cost of implementing an SSR based system is relatively high, the advantages easily outweighs the disadvantages and justifies the cost.